1. Field of the Invention
The invention relates in general to a shift register, and more particularly to a shift register, which has a level controller performing a level control operation by using charging storage ability of capacitors.
2. Description of the Related Art
Along with increasing technology development, nowadays, the liquid crystal display (LCD) has been widely applied to electronic display products, such as TV, computer monitors, notebook computers, mobile phones or personal digital assistants. The LCD includes a data driver, a scan driver and a liquid crystal display panel. The display panel has a pixel matrix. The scan driver is for sequentially enabling the corresponding pixel rows in the pixel matrix for transmitting pixel data outputted by the data driver to the corresponding pixels so as to display the required images.
In today's technology, normally, a shift register is used to implement the function of the scan driver for sequentially enabling the corresponding pixel rows in the pixel matrix and the level controller 100 (in FIG. 1) of each stage of shift register unit is designed by using two transistors differing by a large W/L ratio. As shown in FIG. 1, the W/L ratio of the transistor T5′ is ten times of that of the transistor T4′. The transistor T4′ is biased to be a diode. Therefore, the level controller 100 can generate an output signal Vg having an inverse phase to an input signal Vs in response to the input signal Vs.
However, owing that the transistor T4′ has a smaller W/L ratio, the transistor T4′ has to bear a larger current generated by the transistor T5′, which causes the shift register unit to generate an error operation and shortens the lifetime of the LCD. Therefore, how to design a level controller and shift register with longer lifetime so as to improve the lifetime and frame quality of the LCD is one of the essential targets in the relevant industrial development.